1. Field of the Invention
This invention relates to a continuous and high speed process for the partial hot dipping of long steel strips having areas which require no plating thereon, comprising forming a plating-stopping film on the area and then dipping the long steel strips in a hot-dipping bath.
2. Description of the Prior Art
For the purpose of obtaining corrosion resistance, steel sheets have often been plated with various anticorrosive films, which are generally formed on both sides of the steel sheets. In some cases of, for example, steel sheets for automobiles, where paints are applied after plating the plated surfaces to be additionally painted become inferior in brightness, markedly deteriorating commercial values of the sheets. On the other hand, for instance, galvanized steel sheets have poor weldability because zinc has a small difference between its boiling and melting points these steel sheets have a higher heat conductivity and moreover the welding tends to contaminate welding electrodes. As mentioned above, the application of the anticorrosive film has adverse effects on the appearance after painting and on the weldability. This has resulted in promoting the use of so-called single-side-plated steel sheets in the automotive industry and others. This type of sheet has one side face plated to make it anticorrosive while maintaining good paintability and weldability of the other side face.
An example of hot-dipping process for producing single-side-plated steel sheets (e.g. single-side-galvanized sheets) has been disclosed in Japanese Patent Laid-Open No. 158857/81. The process comprises forming a plating-stopping film consisting of a carbon layer on a steel strip surface at the area which requires no plating. This process is very effectual for single-side plating in that the carbon layer has not only a good plating-stopping function but also can be readily removed off after plating.
The present inventors have discovered that the steel strip surfaces not plated undergo an undesirable oxidation after removing the steel strips from a hot-dipping bath.
When no oxidation-inhibiting film is formed, the following inconvenience takes place, detracting product quality and productivity:
After the formation of a carbon-containing plating-stopping film on one side of a steel strip, the steel strip is dipped in a hot-dipping bath. The plated steel strip is exposed to the atmosphere, where the temperature of the strip initially the same as the hot-dipping bath is cooled naturally. The temperature of the strip just when taken up from the hot-dipping bath is about 450.degree. C. for a zinc hot-dipping bath, about 650.degree. C. for an aluminum hot-dipping bath, and about 330.degree. C. for a tin hot-dipping bath. In all the cases, the strip is exposed at high temperatures to the atmosphere. This causes oxidation of the unplated surface of the strip (the surface of the strip coated with a plating-stopping film) to form an oxide film (scale), when the plating-stopping film does not have the oxidation-inhibiting property. The scale formed on the opposite surface of a single-side plated strip will bring about significant difficulties into such processes as chemical treatments, which are pretreatments for paint application to be made later, and electroplating of this unplated surface. Therefore, there is a need for removing this scale and for adding such steps as acid-cleaning after the plating step of a continuous plating process, thus markedly lowering productivity. When the scale is removed by acid-cleaning, the acid is required to contact with the scale alone because, if the steel strip is simply dipped in the acid, also the plating metal will be dissolved, thereby greatly increasing the cleaning loss; in consequence, the equipment becomes complicated in its construction.
According to the process of the heretofore mentioned Japanese Patent Laid-Open No. 158857/81, a scale that is about 500 .ANG. thick forms during zinc hot-dipping.
When a water glass solution is applied as in the process of the Japanese Patent Laid-Open No. 158857/81, the resulting coating, on being exposed to a high temperature in a heat treatment step in the plating process, foams to become partly porous or develops tortoise shell-like cracks, and oxygen passes through these portions to reach the surface of a steel base, forming a scale. This phenomenon occurs similarly when an aqueous borax solution is applied.
When a silicone resin is applied, the coating is decomposed by heating in a heat-treatment furnace in the plating process, to form SiO.sub.2, and at the same time a volume contraction of the coating take place, thereby developing such defects as cracks in the coating and forming a scale around the defects. In the conventional process, much time is consumed for dissolving and removing the scale, and heating is required for saving time.